Reinforcing fiber materials comprising glass, polymer, other reinforcing fibers, or blends thereof are commonly used as reinforcement materials in molded composites. These reinforcing fiber materials, when incorporated into the matrix resin of such molded composites, provide the finished product with a higher level of tensile strength and durability than could possibly be achieved if either the reinforcing fibers or the matrix resins were used separately. Reinforcing fibers may be incorporated into a composite matrix resin either in continuous form, as is done in the manufacture of filament-wound composites, or, alternatively, the reinforcing fibers may be introduced into the composite matrix resin as chopped segments that may be dispersed throughout the composite matrix in linear or random fashion, depending on the characteristics that are desired in the final molded composite product.
Generally, in the manufacture of reinforcement materials such as preforms that are used in liquid resin molding processes, chopped segments of a reinforcing fiber material, which is also referred to herein as a “fibrous carrier substrate”, may be combined with a binder resin, after which the resulting composition is laid down over a porous form and solidified. The resulting solidified, malted structure is known as a preform. The fibrous carrier substrate is typically of glass, and may, for example, be a glass strand. The preform made from the combination of the binder resin and the glass carrier substrate can then be cured and/or subjected to a liquid resin molding processes to form the molded composite end product.
Several means of combining the binder resin with the glass carrier substrate to make preforms are known in the art. These means include using emulsified binder resin compositions, dry binder resin compositions or molten binder resin compositions in combination with the glass carrier substrate. For example, an emulsion binder composition comprising a heat-curable binder resin in a diluent or solvent may be blended with the glass carrier substrate; or the binder resin and the glass carrier substrate may be combined in a diluent to form a slurry. Either the emulsion or the slurry may then be poured onto a porous form or mold and suction or a vacuum applied to remove the diluent or solvent component, thereby solidifying the preform. The obvious drawbacks associated with using an emulsion binder include the requirement for extensive clean-up of the forming screens; environmental hazards relating to the discharge of solvent or diluent vapors containing volatile organic chemicals (VOCs); risks to the safety of personnel from exposure to such chemicals; and added costs arising from a lengthy drying period or the need for additional equipment to prepare the preform.
Dry binder resin compositions comprising, for example, a powdered binder resin, are also known for use in making preforms. The powdered binder resin is heated sufficiently to melt and cure the binder resin after it is combined with the carrier material. One disadvantage of using the powdered binder resin is that it may be difficult to control the amount of binder powder required to create an acceptable preform. Excess powdered binder resin, when melted, results in the presence of excess molten resin, which may foul equipment and require extensive cleanup operations.
Alternatively, a molten binder resin that is obtained from a source other than a powdered binder resin may be combined with the fibrous carrier substrate to make a preform. To make such a preform using molten binder, typically strands of glass carrier substrate are chopped into segments, which are combined with the binder resin and placed over a porous structural form such as a mesh screen. Alternatively, the strands of glass carrier substrate material are chopped into segments and sprayed over the porous structural form, after which a binder resin is added. The method of adding the binder resin may be via a flame-spray process, in which solid, powdered binder resin is sprayed through a flame immediately before it contacts the glass carrier substrate. In this fashion, the binder resin is melted before it mixes with the glass carrier substrate. A process involving the steps of heating, curing and cooling of the material is then applied to form, shape and consolidate the mixture, as well as to remove any solvents or diluents that may be present. In this manner, the product is solidified into a preform ready for molding or further processing. The solidified preform may then be removed and used in a subsequent molding operation, such as injection molding, in which a molding resin is injected around the preform and the combination is then cured to form a structurally molded composite.
Because this technique of making the preform typically requires applying an excess of binder resin that eventually melts during the manufacturing process, a commonly observed drawback is the build-up of excess molten binder resin on the equipment, the removal of which is both costly and time-consuming. Moreover, the process includes the inherent difficulties of dealing with the molten binder resin. For example, the process of adding the binder resin is difficult to control, and the handling of the hot, molten binder resin poses an additional safety concern.
Continuous glass fiber strands that have been pre-impregnated with a binder resin may also be chopped into fiber strand segments for preform manufacture. The pre-impregnated strands, known as string binders, may be formed by applying one or more layers of a binder resin onto the surface of a continuous glass fiber strand after it is formed, then allowing the binder resin to solidify on the surface of the strand. After the coating of binder resin is solidified, the coated strand is then chopped into string binder segments that may be used in a spray-up process to make preforms.
The binder resins used in preform manufacture are usually either thermoplastic polymers in molten or powdered form, or low acid value thermosetting emulsion polymers such as crystalline polyesters. The term “crystalline” relates to the inherent ability of the thermosetting resin to form crystallites or regions of molecular order dispersed among regions of molecular disorder within the solidified polymer. The ability of a polymer to display crystalline properties is determined principally by its composition. For example, thermoplastic polyesters are macromolecules that contain no chemical groups to effect inter-linking when heated. The absence of such inter-linking defines these polymers as thermoplastic in nature. Such thermoplastic polymers are typically heated to the softening point, forced into the shape of the desired article, then cooled below the softening point to yield the finished reinforcing article. However, like thermosetting polyesters, the thermoplastic polyesters may display many levels of crystallinity, again depending on composition.
Crystalline thermosetting polyesters find use, for example, in organic fiber manufacture. Perhaps the best known crystalline polyester is polyethylene terephthalate, PET, which is commonly known as DACRON polyester, available from DuPont Inc.
The term “high acid value”, as used herein with respect to the binder resin, is intended to represent the acidity of the binder resin, as measured in terms of the amount of potassium hydroxide (KOH) required to neutralize the acidic functional groups in one gram of the binder resin. A high acid binder resin is one that contains acidic functional groups such that the measured acid value of the binder resin is greater than 30 mg KOH/g of binder resin. The known drawbacks of using such high acid binder resins include a high level of incompatibility between the binder resin molecules and the composite matrix resin because of the large degree of difference in polarity between the binder resin molecules and the matrix resin molecules, and/or the absence or unavailability of reactive functional groups that can crosslink with the composite matrix resin. This incompatibility can result in a lesser degree of wet-out of the fibrous carrier substrate in the composite matrix resin. The poor wet-out of the fibrous carrier substrate in turn leads to associated product defects such as blistering during the composite molding phase, and bleeding or blistering during post-bake of the molded composite product.
Bleeding in the molded composite product is related to certain characteristics of the binder resin that affect its compatibility with the matrix composite resin. While thermoplastic and thermosetting resins have both been used as a binder resin in string binder formulations, the different characteristics of these types of polymers affect their use in composite formulations. Where the binder resin is a thermosetting polymer, a resin with a lower molecular weight may generally be used because the molecules will link during cure to form a permanently solidified, continuous, cured matrix with essentially infinite molecular weight. The lower molecular weight resin will easily flow and therefore will more fully coat the fibers of the fibrous substrate. Typically, such thermosetting binder resin polymers are thermosetting crystalline polyester resins made up of small molecules, which melt and flow easily. In contrast, molecules of thermoplastic binder resin do not link to form a permanently solidified matrix. Rather, the solidified matrix may be induced to re-melt and flow by applying heat. In order to achieve acceptable performance using a thermoplastic binder resin, it is typically necessary to begin with thermoplastic resins that have a higher molecular weight. Such thermoplastic resins are usually composed of long chains of atoms, which become easily entangled, thereby causing a restriction of flow. This reduced flow, which results in a higher melt viscosity, is a disadvantage in that it impedes flow of the coating over the fibers. Further, the large, unlinked thermoplastic resin molecules demonstrate a tendency to diffuse through the composite matrix during post-baking. This diffusion or bleeding typically causes blemishes in the surface of the composite.
Blistering may result from an undesirable chemical reaction between a component of the composite matrix resin and the binder resin during the composite curing process. For example, where the composite matrix resin is a polyurethane, an isocyanate group of the polyurethane may react with acid or water in the binder to form carbon dioxide and an amine as reaction by-products. The evolution of the carbon dioxide gas can lead to the formation of blisters on the surface of the cured composite. Blistering may ultimately result in decreased glass/matrix resin bond strength in the preform-reinforced composite, and, as a result, the physical strength of the finished, molded composite article may be diminished. Blistering is also aesthetically undesirable because the appearance of the molded composite product is compromised.
There is, therefore, a need for a manufacturing process, for example for making preforms to be used in composite molding, in which a fibrous carrier substrate may efficiently be combined with a binder resin before molding the preform, such that the separate application of a binder resin in the form of a powder melt, emulsion or slurry is not required. Further, there exists a need for a moldable structure comprising a fibrous carrier substrate and a binder resin, which enhances wet-out and prevents undesirable effects such as blistering or bleeding when the combination comprising the fibrous carrier substrate and binder resin are used in a composite molding process. There also exists a need for a combination of ingredients for making a preform that does not rely on the use of environmentally hazardous organic solvents, or other solvents that require a drying procedure that lengthens the manufacturing ,process. These needs are met by the invention described herein.